The present invention relates to the in situ leaching of mineral values and more particularly relates to a method and apparatus for mixing a gaseous oxidant and a lixiviant at a downhole location for use in an in situ leach operation.
In a typical in situ leach operation, wells are completed into a leachable mineral-bearing formation and a lixiviant is flowed between wells to dissolve the mineral values into the lixiviant. The pregnant lixiviant is produced to the surface where it is treated to recover the mineral values from the lixiviant.
Many leachable mineral values as they occur in their natural state in a formation must be oxidized to a higher valence before they become soluble into a lixiviant. For example, uranium is normally present in a formation in the tetravalent state and must be oxidized to the hexavalent state to render it soluble in a suitable lixiviant, e.g., an aqueous carbonate solution. To oxidize uranium to its higher valence, it is customary to contact the uranium in the formation with an oxidant which may be injected directly into the deposit or which may be mixed into the leach solution and injected therewith.
Several oxidants have been proposed for this purpose, including air and oxygen. For example, in U.S. Pat. No. 3,708,206, oxygen is injected into a formation prior to or simultaneously with a lixiviant. In U.S. Pat. No. 3,713,698, air is injected through a production well to oxidize uranium values prior to injecting a lixiviant through an injection well. In both U.S. Pat. Nos. 3,640,579 and 3,860,289, oxygen is supplied through a tube to a downhole location where it is bubbled into a lixiviant before the lixiviant is injected into a formation.
With each of these types of injection schemes, excess quantities of oxygen are required. For example, where oxygen is merely bubbled into the lixiviant downhole before the lixiviant enters the formation, experimentation suggests that a tenfold to fiftyfold excess of oxygen over the saturation requirement is needed, resulting in excessive oxygen costs. Of course, this excess oxygen could be collected, recompressed, and recycled; but the cost of doing this is equally as excessive.
Another approach in mixing oxygen and lixiviant downhole might be to use a mechanical agitator (beater) downhole. However, the high capital investment, along with operational and maintenance costs, makes such an approach impractical. Still another approach is to inject oxygen through fine frits or spargers located downhole to form small bubbles in the lixiviant to effect a good mass transfer of oxygen to the lixiviant. However, based on known in situ leach conditions, precipitates present in the leach operations will likely plug the frits quickly thereby severely restricting the necessary oxygen flow.
Therefore, the method used to mix oxygen or other gaseous oxidants with a lixiviant at a downhole location to saturate the lixiviant before injection into a formation should be simple and rugged in that the apparatus used should (1) not be susceptible to plugging, either by materials carried in the lixiviant or by precipitates resulting from chemical reactions of the oxygen with materials in the lixiviant, (2) be able to be run into and out of the hole as a unit, preferably on the end of the oxygen injection conduit, and (3) be effectively self-controlling, i.e., not requiring complex controls at either the surface or downhole. The method should also be efficient in that (1) substantially no free oxygen gas is allowed to bubble up against the downflowing lixiviant and collect in surface connections and (2) no additional energy sources such as power for motor-driven downhole mixers, etc., are required. Further substantial amounts of free or undissolved oxygen, if permitted to enter the formation with the lixiviant may adversely affect the efficiency of the leach operation. Therefore, some means should be provided to control the amount of free oxygen in the saturated lixiviant before it is injected into the formation.